To reduce dissipation losses in the output stage, most audio frequency amplifiers comprise a class B or AB amplifier. A conventional class B or AB amplifier comprises a pair of emitter-coupled output transistors configured in a push-pull arrangement. The audio frequency signal is applied to the bases of these output transistors and the amplified audio frequency signal is present at the emitters thereof. In a conventional push-pull amplifier, the collector of one of the output transistors is connected to a fixed positive DC voltage to provide a current source and the collector of the other of the output transistors is connected to a fixed negative DC voltage to provide a current sink. The positive and negative DC voltages are commonly referred to as the source and sink voltages, respectively.
With a reactive load, the entire fixed voltage is present across the amplifying transistor of a class B or AB amplifier when current flows to the load, yielding high dissipation losses. These losses are particularly high with high dynamic range signals such as music signals. Accordingly, while more efficient than a pure class A amplifier, class B and AB amplifiers still exhibit significant dissipation losses with resistive loads and exhibit considerably greater losses with reactive loads. At the high power levels of modern audio amplifiers, these dissipation losses require the use of numerous expensive high power semiconductors in parallel and also that extensive steps be taken to cool the output transistors.
To obtain higher efficiencies, class D amplifiers have also been proposed as audio amplifiers. Class D amplifiers comprise a power transistor, a low pass filter, and a freewheeling diode or rectifier in parallel with the low pass filter. The power transistor is switched on and off according to a high frequency square wave signal the pulse width of which is modulated according to the audio frequency signal to be amplified. The filter then recovers the audio frequency signal by filtering off the high frequency square wave signal. The class D amplifier achieves high efficiencies by delivering current only when substantially zero volts are present across the power transistors.
While significantly more efficient than a class B or AB amplifier, a class D amplifier has high distortion that cannot be easily be corrected with negative feedback because of the phase shift introduced by the low pass filter. Further, the low pass filter tends to interact with the load in an undesirable fashion. Also, because of the relatively high frequencies involved, class D amplifiers are subject to radiation problems. Finally, class D amplifiers exhibit poor power supply rejection and thus are highly susceptible to power supply disturbances. For these and other reasons, class D amplifiers have not been used in commercial audio amplifiers.
Two other classes of high efficiency audio frequency amplifiers have been proposed to increase the efficiency of output transistors arranged in a push-pull configuration. These amplifiers are referred to as class G and class BD amplifiers and employ adaptive power supplies for generating source and sink voltages for a push-pull amplifier. These adaptive power supplies generate source and sink voltages that increase and decrease as the audio frequency signal increases and decreases. The basic idea with these amplifiers is to provide high voltage to the push-pull amplifier only when the audio frequency signal is high. As an audio frequency signal developed from a musical source is normally relatively low with infrequent high bursts, class G and BD amplifiers used as audio amplifiers normally maintain the voltage across the power transistors at a low level, thereby greatly reducing the average power dissipation of the output transistors.
A class G amplifier normally comprises a push-pull amplifier and a stepped power supply that generates source and sink voltages that are increased and decreased in two or three discrete steps as the power requirement of the signal being amplified increases and decreases. Such an amplifier is disclosed, for example, in U.S. Pat. No. 4,484,150 to Carver and U.S. Pat. No. 3,961,280 to Sampei.
A class BD amplifier conventionally comprises a pair of highly efficient class D amplifiers to provide signal tracking source and sink voltages to a push-pull amplifier. Class BD amplifiers are generally discussed in the following articles: (a) The Class BD High-Efficiency RF Power Amplifier dated June 1977 and written by Frederick H. Raab; and (b) An Amplifier With A Tracking Power Supply dated Nov. 5, 1973, and written by V.M. Kibakin.
The present invention is particularly useful when implemented in the context of amplifiers containing tracking power supplies. Tracking power supplies can be classified as envelope trackers, rail-to-ground trackers, and rail-to-rail trackers. The present invention may be used to advantage in each of these configurations, but is of particular importance when used with rail-to-rail trackers, as will be discussed in detail below.
Exemplary envelope trackers are disclosed in U.S. Pat. No. 3,426,290 issued Feb. 4, 1969 to Jensen, U.S. Pat. No. 4,218,660 issued Aug. 19, 1980 to Carver, and, more recently, U.S. Pat. No. 5,075,634 issued Dec. 24, 1991 to French.
Rail-to-ground tracking power supplies are disclosed in U.S. Pat. No. 4,054,843 issued Oct. 18, 1977 to Hamada, U.S. Pat. No. 4,409,559 issued Oct. 11, 1983 to Amada, an article published by the Audio Engineering Society in 1981 entitled A HIGH EFFICIENCY AUDIO POWER AMPLIFIER (Nakagaki and Amada), and U.S. Pat. No. 4,507,619 issued Mar. 26, 1985 to Dijkstra.
Rail-to-rail tracking power supplies are disclosed in U.S. Pat. No. 4,087,759 issued May 2, 1978 to Iwamatsu, U.S. Pat. No. 4,472,687 issued Sep. 18, 1984 to Kashiwagi et al., and U.S. Pat. No. 5,200,711 issued Apr. 6, 1993 to Andersson et al.
In an amplifier having any one of these three types of tracking power supplies, the collector-emitter voltage across the amplifying transistor will ideally remain substantially constant at a low value. The basic advantage of all types of tracking power supplies is thus that dissipation losses in the amplifying transistor are reduced. Additionally, in tracking power supplies, dissipation losses of the amplifying device are ideally kept low for both resistive and reactive loads.
A tracking power supply in a class BD amplifier will thus in general reduce by varying degrees the dissipation of the output transistors relative to the dissipation of the output transistors in a pure class B or AB amplifier. Rail-to-rail tracking power supplies are theoretically the most efficient, while envelope tracking power supplies are the lease efficient of the three types of tracking power supplies. Rail-to-ground and rail-to-rail tracking power supplies will also have the additional advantage of reducing the voltage rating requirements of the output transistors.
As mentioned, in theory the most efficient of the various amplifier configurations having tracking power supplies is the rail-to-rail tracking power supply. However, despite the potential advantages theoretically obtainable by using a rail-to-rail tracking power supply, no commercially available amplifier exists that uses a rail-to-rail tracking power supply as described above.